Leaky wave antennas are electromagnetic traveling-wave radiators receiving a feed signal at one end and terminated in a resistive load at the other. The feeding end is used to launch a wave that travels along the antenna while leaking energy into free space. Power remaining in the traveling wave as it reaches the antenna end is absorbed by the resistive load. Using a single feed signal to excite a leaky-wave antenna results in higher radiation efficiency than in a microstrip antenna array. This is because microstrip antenna arrays suffer from spurious radiation and ohmic losses associated with their corporate feed. The aforementioned features of leaky-wave antennas make them well suited for operation at high frequencies.
In 1979, Menzel introduced a traveling-wave microstrip antenna based on the first higher-order mode (EH1) (W. Menzel, “A new traveling-wave antenna in microstrip”, Arch. Elektron. Ubertragungstech., vol. 33, no. 4, pp. 137-140, April 1979). The antenna was asymmetrically fed by means of a microstrip line as shown in FIG. 1a and FIG. 1b. Transverse slots located along the center line of the antenna were used to suppress the fundamental mode. Using a quarter-wave transformer, the input impedance of the antenna was matched to the characteristic impedance of the microstrip feed line. The antenna radiated an x-polarized main beam at an angle θ=37.5° away from broadside (the z direction). The antenna exhibited an impedance bandwidth broader than that of the resonant microstrip patch, but also produced a high backlobe level.
Oliner and Lee later disclosed that the microstrip antenna introduced by Menzel could be operated as a leaky-wave antenna had it been configured to be 4.85 λ0 long instead of 2.23 λ0, where λ0 is the free space wavelength at the design frequency (A. Oliner and K. S. Lee, “The Nature of the Leakage from Higher Modes on Microstrip Line”, 1986 IEEE International Microswave Symposium Digest, and “Microstrip Leaky-Wave Strip Antennas”, 1986 IEEE International Antennas and Propagation Symposium Digest). They also disclosed that Menzel's antenna exhibits a high backlobe level because 35% of the incident power is reflected at the terminated end, with the backlobe appearing at the same angle as the main beam when measured from the broadside. A three-dimensional view of Oliner and Lee's leaky-wave microstrip antenna is shown in FIG. 2.
The amplitude of the x-directed current traveling along the aforementioned leaky-wave microstrip antenna is shown in FIG. 3a. It is an exponentially decaying distribution that results in the x-polarized H-plane power-gain pattern shown in FIG. 3b. This pattern exhibits sidelobes that appear at ˜12 dB below the main beam, and are undesirable in applications such as radar since they result in false-target identification. What is needed is an efficient leaky-wave microstrip antenna having high gain and low side lobes.